Populations of Alouatta seniculus are being studied in two separate geopraphical areas in northern Venezuela-the submontane rainforests in Guatopo National Park and the llanos habitat, approximately 55 km. south of Calabozo, in the State of Guarico, Venezuela. In the llanos study site, two separate areas are under intensive investigation: (1) where the red howler monkey occurs as the only primate species and (2) at the study site in the gallery forests where it occurs in sympatry with Cebus nigrivittatus. Some 38 individual animals have been marked. The work has been in progress since 1975. We are studying by advanced quantitative methods, feeding behavior and movement patterns. In addition, the demography of the various populations is being analyzed. Population turnover, emigration and natality have been givec special attention. The completed study should elucidate problems of carrying capacity for natural primate populations. The howler monkey, Alouatta seniculus can serve as an indicator species in assessing environmental quality for neotropical primate populations.